396 



Minnesota Plant Diseases. 



site on many wild plants and is also found on cultivated plants. 

 It is found in great abundance on plants of the crowfoot and 

 pea families. It appears on wild vetches as an extensive, fine, 

 white mycelium, which bears the summer spores as a starchy 

 powder, typical for the powdery mildews. The spore-sac-cap- 

 sules are formed in the late summer and fall and appear as small 

 black bodies about the size of a very small pin-point. The cap- 

 sules have unbranched, brown appendages, which are inter- 

 woven with the mycelial hyphae. They contain a number of 

 sacs, each enclosing eight spores. This mildew, though com- 

 mon on wild plants, is apparently not very destructive to culti- 

 vated plants. 



If necessary, 

 treatment as 

 against powdery 

 mildews in gen- 

 eral would prob- 

 ably be effective. 

 Powdery mil- 

 dew of compos- 

 ites (Erysiphe ci- 

 choracearum DC). 

 This is perhaps 

 the most com- 

 mon of all our 

 powdery mildews. 

 It is exceedingly 

 abundant upon a 

 great number of 

 wild plants, be- 

 longing chiefly 

 to the composite 

 family. The bo- 

 rage and other 

 families are less 

 frequently at- 

 tacked. It is 

 found on s u n- 

 flowers, rag-weeds, verbenas and a host of other plants. It 

 forms a more or less dense and conspicuous, white, cottony 



Fig. 210. — Powdery mildew of composites, on the leaf of the 

 great ragweed. The white felt of the superficial mycelium 

 is shown and the numerous black dots are the sac-spore- 

 capsulcs. Original. 



